1. Field of the Invention
The invention relates to a road surface friction state estimating apparatus. More particularly, the invention relates to a road surface friction state estimating apparatus which estimates a road surface friction state using self aligning torque generated in a tire of a vehicle, and a method thereof.
2. Description of the Related Art
As related art 1 of the invention, Japanese Patent Laid-Open Publication No. 6-221968 discloses a road surface friction coefficient detecting apparatus which estimates a grip state using self aligning torque (hereinafter, referred to as “SAT”).
The apparatus according to the related art 1 estimates a grip state based on SAT and a front wheel lateral force which are calculated based on vehicle movement. According to the related art 1, it is possible to accurately estimate a grip state even when the vehicle runs on a bank road surface, or when a relation between a lateral force and a slip angle is non-linear.
However, the apparatus according to the related art 1 is likely to be influenced by road surface disturbance. Also, when steering is performed quickly, the phase of the calculated SAT value and that of the calculated lateral force value are deviated from each other. Accordingly, a grip state cannot be accurately estimated.
As another related art 2 of the invention, Japanese Patent Application No. 2001-212683 discloses art in which a front wheel slip angle is estimated based on a steering angle, SAT is estimated by removing friction of a steering system from a sum of steering torque supplied by a driver and assist torque applied to a power steering device, and a grip state is estimated based on the front wheel slip angle and SAT.
However, according to the related art 2, a grip state cannot be accurately estimated when the relation between the lateral force and the slip angle becomes non-linear and accordingly the front wheel slip angle cannot be accurately estimated in the case where the steering angle is 0 and SAT is not 0 during running on a bank road surface.